what do you want to mod exactly? There are modding tools to edit a lot of things, plus Playbills scripting which is new to the COH3 essence engine... :)

1. I want to change the way speed of the infantry.
2. Give the infantry the ability to call in airstrikes and artillery.
3. Retexture the infantry.

Hello. All depends on your knowledge of essence editor. But to answer one by one.

1. yep : ebps, moving
2. yep : many steps to do, but by cloning abilities, then giving on sbps section the abilitie you want (cautious many requirements to clear, ui button with right row/column, etc...)
3. some data are not allowed to be cloned, which is my super problem. In your case, with essence editor, you gonna have troubles to do it without these access. My level is not enough to help atm.